The present invention relates to a magnetoresistive effect element capable of producing a magnetoresistive change by a current flowing to the direction perpendicular to the film plane and a magnetic memory device including this magnetoresistive effect element.
As information communication devices, in particular, personal small devices such as personal digital assistants are making great spread, elements such as memories and logic devices comprising information communication devices are requested to become higher in performance such as becoming higher in integration degree, higher in operation speed and becoming lower in power consumption. In particular, technologies for making nonvolatile memories become higher in density and larger in storage capacity are progressively increasing their importance as technologies for replacements for hard disks and optical discs that cannot be essentially miniaturized because the hard disks and the optical discs cannot remove their movable portions.
As nonvolatile memories, there may be enumerated a flash memory using a semiconductor and an FRAM (Ferro electric Random Access Memory) using a ferroelectric material and the like. On the other hand, a problem has been pointed out, in which the FRAM cannot be rewritten so many times.
As a nonvolatile memory that has received a remarkable attention because it can overcome these shortcomings, there is known a magnetic memory called an MRAM (Magnetic Random Access Memory) which had been written in “Wang et al., IEEE Trans. Magn. 33(1997), 4498”. Since this MRAM is simple in structure, it can be easily integrated at a higher integration degree. Moreover, since it is able to record information based upon the rotation of magnetic moment, it can be rewritten many times. It is also expected that the access speed of this magnetic random access memory will be very high, and it was already confirmed that this magnetic random access memory can be operated at operation speed in the order of nanoseconds.
A magnetoresistive effect element for use with this MRAM, in particular, a tunnel magnetoresistive effect (Tunnel Magnetoresistive: TMR) element has a fundamental arrangement of a lamination layer structure composed of ferromagnetic layer/tunnel barrier layer/ferromagnetic layer. This element generates magnetoresistive effect in response to a relative angle between the magnetizations of the two magnetic layers when an external magnetic field is applied to the ferromagnetic layers under the condition in which a constant current is flowing through the ferromagnetic layers. When the magnetization directions of the two ferromagnetic layers are anti-parallel to each other, a resistance value is maximized. When they are parallel to each other, a resistance value is minimized. This magnetic random access memory can function as the memory element when the anti-parallel state and the parallel state are created with application of external magnetic fields.
In particular, in a spin-valve type TMR element, when one ferromagnetic layer is antiferromagnetically coupled to the adjacent antiferromagnetic layer, one ferromagnetic layer is served as a magnetization fixed layer of which the magnetization direction is constantly made constant. The other ferromagnetic layer is served as a magnetization free layer of which the magnetization direction can easily be inverted with application of external magnetic fields and the like. Then, this magnetization free layer serves as an information recording layer in the magnetic memory.
In the spin-valve type TMR element, the changing ratio of the resistance value may be expressed by the following equation (1) where P1, P2 represent spin polarizabilities of the respective ferromagnetic layers.2P1P2/(1−P1P2)   (1)
As described above, the resistance changing ratio increases as the respective spin polarizabilities increase.
With respect to relationships among materials for use with the ferromagnetic layers and this resistance changing ratio, those relationships concerning ferromagnetic chemical elements of Fe group such as Fe, Co, Ni and alloys of these three kinds have been reported so far.
As is disclosed in Japanese laid-open patent application No. 10-116490 and the like, for example, the MRAM has a fundamental arrangement comprising a plurality of bit write lines (so-called bit lines), a plurality of word write lines (so-called word lines) perpendicular to those bit write lines and TMR elements disposed at intersection points between these bit write lines and word write lines as magnetic memory elements. Then, when information is written (information is recorded on) in such MRAM, information is selectively to be written in the TMR element by utilizing an asteroid property.
As the write line for writing information, there is used a conductive thin film, such as Cu and Al, which is generally used in semiconductors. For example, in order to write information in an element of which inverted magnetic field is 200 Oe by a write line having a width of 0.25 μm, a current of about 2 mA was required. When the thickness of the write line is identical to the line width, a current density obtained at that time reaches 3.2×106 A/cm2 and reaches approximately a limit value of breaking wire by electromigration. Due to a problem of heat generated by a write current and from a standpoint of reducing power consumption, it is necessary to decrease this write current.
As a method of realizing reduction of the write current in the MRAM, there may be enumerated a method of decreasing a coercive force of the TMR element. The coercive force of the TMR element may properly be determined based upon selection of suitable factors such as the dimension and shape of the element, the film arrangement and the material.
However, when the TMR element is microminiaturized in order to increase the recording density of the MRAM, for example, a disadvantage arises, in which the coercive force of the TMR element increases unavoidably.
Therefore, in order to microminiaturize the MRAM (to increase the integration degree of the magnetic random-access memory) and to decrease the write current at the same time, the decrease of the coercive force of the TMR element should be attained from the materials standpoint.
If the magnetic property of the TMR element is dispersed at every element in the MRAM and the magnetic property is dispersed when the same element is in repeated use, then a problem arises, in which it becomes difficult to selectively write information in the magnetic random-access memory by using the asteroid property.
In consequence, the TMR element is requested to have a magnetic property by which an ideal asteroid curve can be drawn.
In order to draw the ideal asteroid curve, an R-H (resistance-magnetic field) curve obtained when the TMR is measured should remove noises such as a Barkhausen noise, a rectangle property of a waveform should be excellent, the magnetization state should be stable and the dispersions of the coercive force Hc should be small.
On the other hand, information will be read out from the TMR element in the MRAM as follows. When magnetic moments of one ferromagnetic layer and the other ferromagnetic layer across the insulating layer are anti-parallel to each other and the resistance value is high, this state is referred to as a “1”, for example. Conversely, when the respective magnetic moments are parallel to each other, this state is referred to as a “0”. Then, it is customary to read out information from the tunnel magnetoresistive effect element based upon a difference current between these states by using a constant voltage source.
Therefore, when the dispersions of resistance values between the elements are identical to each other, higher the TMR ratio (magnetoresistive changing ratio) becomes, the tunnel magnetoresistive effect element becomes more advantageous. Thus, the magnetoresistive effect element which can operate at high speed, may have a high integration degree and which may have a low error rate can be realized.
A bias voltage dependence of a TMR ratio exists in the TMR element having the fundamental structure of ferromagnetic layer/tunnel insulating layer/ferromagnetic layer, and it is known that the TMR ration decreases as the bias voltage increases. Since it is known that the TMR ratio takes the maximum value of the read signal near a voltage (Vh) halved by the bias voltage dependence in most cases, a small bias voltage dependence is effective for decreasing read errors.
In consequence, the TMR element for use with the MRAM should satisfy the above-mentioned write characteristic requirements and read characteristic requirements at the same time.
However, when the materials of the ferromagnetic layers of the TMR ratio are selected, if the alloy compositions by which the spin polarizabilities shown by P1 and P2 in the equation (1) are increased are selected from the materials made of only ferromagnetic transition metal chemical elements of Co, Fe, Ni, then the coercive force Hc of the TMR element generally tends to increase.
When the magnetization free layer (free layer), i.e., information recording layer is made of a Co75Fe25 (atomic %) alloy and the like, for example, although the spin polarizabilities may be large and a TMR ratio of 40% or higher can be maintained, it is unavoidable that the coercive force Hc also increases.
When the information recording layer is made of an Ni80Fe20 (atomic %) alloy which is what might be called a permalloy known as a soft magnetic material and the like instead of the above-mentioned alloy, although the coercive force Hc can be decreased, the spin polarizabilities are low as compared with those of the above-mentioned Co75Fe25 (atomic %) alloy so that the TMR ratio is lowered up to approximately 33%.
Further, when the information recording layer is made of a Co90Fe10 (atomic %) alloy having an intermediate property between those of the alloys of the above-mentioned two compositions, although a TMR ratio of about 37% can be obtained and the coercive force Hc can be suppressed to approximately an intermediate value between the coercive force of the above-mentioned Co75Fe25 (atomic %) alloy and the coercive force of the above-mentioned Ni80Fe20 (atomic %) alloy, the magnetoresistive effect element has a poor rectangle property of an R-H loop, and an asteroid property by which information can be written in the magnetoresistive effect element cannot be obtained.
In order to solve the above-mentioned problems, it is an object of the present invention to provide a magnetoresistive effect element having a satisfactory magnetic property and a magnetic memory device including this magnetoresistive effect element and which has excellent write/read characteristics.